Audio Filter
by Steve Hunt

Low pass and High pass audio filters can be realised easily and at low cost using resistors and capacitors in the feedback networks around operational amplifiers. AudFilt takes all the hard work out of calculating the component values required to realise a filter with a particular cut-off frequency. It assumes that a 3-pole Butterworth filter response is required.

Tutorial
Double-click on the AudFilt icon in the filer window; the program will be loaded, and a filter response icon will be installed on the Desktop icon bar. Click Menu over this icon and the usual Info window and Quit options are available. Click Select over the icon and the Audio Filter window will open. Let's suppose that we want to design a high pass 3-pole Butterworth filter with a cut-off frequency of 3KHz, and that we would prefer to use readily available component values if at all possible. You will see that the circuit diagram currently shown is for a low pass filter, so click Menu and select the High pass option. The circuit diagram will change to the new configuration.

Now change the value in the Frequency field from 1000Hz to the required cut-off frequency, 3000Hz. You can do this either by using the up and down arrow icons each side of the field, or by clicking over the field and editing the current value. If you use the latter method, press Return finally to force a recalculation.

Our engineering experience suggests that the resistor values shown (R1=2.6K, R2=3.1K and R3=33.5K) are a little on the low side for this type of circuit, so we will reduce the capacitor value in order to force a corresponding increase in the resistor values. Click over the down arrow icon to the right of the Capacitor value field. The capacitor value will decrease from 8.2nF to 6.8nF and there will be a corresponding increase in the calculated resistor values.

Continue decreasing the capacitor value until it reads 2.2nF. Notice how the capacitor values are restricted to preferred values. The resistor values are now more "sensible" for this type of circuit, but they are not preferred values (ie. not easily obtainable). We are now interested to see what change in cut off frequency would be caused if we used preferred value resistors. We shall do this by changing the frequency in small amounts until the resistor values are close to preferred values.

If we increment or decrement the frequency at the moment it will change in 500Hz steps which is too course an adjustment  50 Hz would be more appropriate. So click Menu, move right over the Freq steps option, and move the pointer to the bottom field in the Step (Hz) menu. Press Ctrl-U to clear the current value of 500, and enter a new value of 50; then press Return.

Now click over the down arrow icon to the right of the Frequency field; the frequency will change to 2950Hz. We now see that the resistor values are 10K, 11.6K and 126.9K. These values are close enough to the available E12 series preferred values of 10K, 12K and 120K for us not to bother further. We decide that we are happy with the present values and would like a print out of the results.

Assuming that you have loaded a RISC OS printer driver, click Menu, move right over the Save option and a standard Save as: window will appear with a default file name of FilterData. If you wish to save the information you can drag the icon to a filer window; in our case we drag the icon to a printer driver icon on the iconbar and printing will begin.

Further details
In the Low Pass mode, resistor values are specified in the component field, and AudFilt calculates the required capacitor values. In the High Pass mode, capacitor values are specified and AudFilt calculates resistor values.

In the Low Pass mode, resistor values are restricted to the E12 range 100 Ohms to 820 KOhms. In the High Pass mode, capacitor values are restricted to the E12 range 100pF to 820nF.

Cut-off frequencies are restricted to integers in the range 10 Hz to 100 KHz.

 Copyright RISC User 1994